Continuous solution polymerization processes generally involve the addition of catalyst to a monomer and solvent mixture. The mixture may be back-mixed giving a uniform polymer in an environment with substantially no concentration gradients. WO 94/00500 (Pannell et al.), incorporated herein by reference, describes a solution polymerization using metallocene in a continuous stirred tank reactor, which may be in a series reactor arrangement to make a variety of products.
For the purposes of this patent specification the term “metallocene” is herein defined to contain one or more cyclopentadienyl moiety in combination with a transition metal of the Periodic Table of Elements.
The heat of the polymerization reaction, called an exotherm, can be absorbed by the polymerization mixture. Alternatively, or in addition, the heat of reaction can be removed by a cooling system, by external cooling of the walls of the reactor vessel, or by internally arranged heat exchange surfaces cooled by a heat exchange fluid.
In the course of the polymerization, typically, a predominant amount (over 50 mol %) of the monomer is consumed and the polymer formed is dissolved in the solvent. The higher the concentration of the polymer, the higher the viscosity of the polymerization reaction mixture containing the polymer, solvent, and unreacted components. The mixture passes from the polymerization reactor to a finishing section in which polymer, solvent and unreacted monomer are separated. In the course of finishing, solvent and unreacted monomer are progressively removed from the polymerization mixture until the polymer can be formed into a solid pellet or bale. The separated solvent and monomer can be recycled to the polymerization reactor.
It is well known from extensive literature sources that polymer solutions can undergo phase separation at the lower critical solution temperature, with phase separation being encouraged by higher temperatures and/or by lower pressures. Solvents selection also influences the conditions where phase separation occurs.
The phenomenon of phase separation is firstly a consideration in the selection of the polymerization solvent. Appropriate polymerization monomer conversions, especially of the volatile monomers, temperatures, and pressures have to be selected for given polymer/solvent combination conditions to avoid unwanted phase separation inside the reactor. Solvents such as hexane may require an elevated pressure in excess of 50 bar to avoid two-phase conditions for olefin polymerization; solvents such as octane can maintain homogeneous one-phase conditions at lower pressures.
The phenomenon of phase separation can secondly be exploited after the reaction step to separate volatile solvent and unreacted monomer components on the one hand, and polymer on the other hand. In that case, separation at temperature well above the lower critical solution temperature is encouraged to allow the polymer to form a concentrated phase. Some earlier articles explain the general principles of which we are aware are: “A Low-Energy Solvent Separation Method,” by T. G. Gutowski et al, Polymer Engineering and Science, March 1983, Vol. 23, No. 4, pp. 230-237; “Lower Critical Solution Temperature Behavior of Ethylene Propylene Copolymers in Multicomponent Solvents” by C. A. Irani et al. in Journal of Applied Polymer Science (1986), Vol. 31, pp. 1879-1899; “Separating Polymer Solutions with Supercritical Fluids,” by Mark A. McHugh et al. in Macromolecules 1985, Vol. 18, pp. 674-680; “Critical dynamics and phase separation kinetics in dynamically asymmetric binary fluids: New dynamic universality class for polymer mixtures or dynamic crossover?,” by Hajime Tanaka in Journal of Chemical Physics 1 Apr. 1994, 100 (7), pp. 5323-5337; “Short Chain Branching Effect on the Cloud Point Pressures of Ethylene Copolymers in Subcritical and Supercritical Propane,” by S. J. Han et al. in Macromolecules 1998, Vol. 31, pp. 2533-2538, each of which is incorporated herein by reference.
The finishing section may also comprise a vacuum devolatilizer, in which the molten polymer is exposed to a vacuum while being intensively agitated to draw off volatiles such as solvent and residual monomer, in order to reduce the level of volatiles in the finished polymer to a desired level.
U.S. Pat. No. 6,881,800 and U.S. Pat. No. 7,163,989, both of which are incorporated herein by reference, describe a process and apparatus for the continuous solution polymerization of olefins including ethylene, propylene and other olefin comonomers. The polymerization reaction takes place under pressure in one or more polymerization reactors, and then the effluent from the reactor or reactors is treated in a finishing section with a catalyst killer and then heated in one or more heat exchangers before being subject to a pressure drop which causes the effluent to phase separate into a polymer-rich phase and a polymer-lean phase. Those phases are separated, with the polymer-lean phase being purified and recycled to be used as solvent. The polymer-rich phase is subject to further separation and purification stages, including passage through a vacuum devolatilizer. Following the vacuum devolatilization, the polymer is formed into pellets and/or bales for storage or shipping. The process is suitable for the manufacture of a range of different polymer types.
While the single site, metallocene catalysts have a high activity; that activity is often sustained under conditions in which phase separation would occur at elevated temperatures. Continued polymerization activity during phase separation may influence polymer characteristics undesirably.
Many types of catalyst are known for olefin polymerization, including Ziegler-Natta, chromium catalysts and single site catalysts such as metallocenes. The use of single site catalysts is associated with poor solubility in the aliphatic hydrocarbon, saturated, non-polar solvents used for homogeneous solution polymerization. As a result, an aromatic catalyst solvent, such as toluene may have to be used. This in turn can complicate solvent separation to prevent toluene build up in the reactor, and lead to environmental pollution and added maintenance expenditure.
In some solution processes (see WO 98/02471 Kolthammer), incorporated herein by reference, the polymerized mixture is flashed off in two stages, whereby the solvent and unreacted monomer are converted to a vapor phase. Efficient extraction of solvent, etc., requires low vapor pressures and vapor phase compression or condensation followed by pumping for subsequent separation stages. Pumping is used to convey polymer from flash separation stages to a final devolatilizing extruder.
U.S. Pat. No. 3,912,698, incorporated herein by reference, uses a heat exchanger for a liquid recycle stream to permit an increase in reactor capacity while reducing fouling in the context of a multiple flash to remove volatiles.
Polymers prepared using continuous solution polymerization have found application as films. In such end uses, it is especially important to minimize the gel content of the polymer because gels give rise to imperfections in the films.
A stabilizer may be added to a polymer during manufacture or processing of a polymer to prevent oxidation on storage. A variety of stabilizers is available commercially.
There remains a need for an improved continuous solution process and plant which provides one or more of the following benefits: producing polymer economically across a broad range of operating windows including varying polymerization temperatures; producing a broad spectrum of polymers, particularly polymers of widely varying average molecular weights, molecular weight distributions, and/or comonomer contents; permitting the production of polymers having useful molecular weights at high temperatures (above 150° C.); accommodating a broad range of catalyst performance; reducing energy consumption, especially in finishing, and reducing environmental discharge; and reducing or avoiding fouling in the recycle and purification systems while using highly active metallocene type catalysts with unreacted monomer and temperature during separation processes.
It would be particularly useful to provide a process and plant which can introduce stabilizer into a polymer in an effective and efficient way, with minimal requirement for additional equipment and minimal operating costs.
For additional background, see also WO 94/00500 and WO 92/14766, incorporated herein by reference.